Typical electrophotographic image forming apparatuses include a cleaner that removes residual toner particles from an image bearing member. Those employing an elastic blade as the cleaner are well-known. The elastic blade (hereinafter “cleaning blade”) is typically pressed against a peripheral surface of the image bearing member to scrape off residual toner particles. The above system, what is called a blade cleaning system, is widely used because of having a simple configuration and stable performance. On the other hand, toners have been developed to be much smaller and more spherical to meet a recent demand for higher image quality. Small-sized toners are advantageous in producing high-definition and high-resolution images, and spherical toners are advantageous in improving development efficiency and transfer efficiency.
However, it is difficult for the blade cleaning system to remove small-sized and spherical toner particles for the following reasons. An edge of the cleaning blade is in frictional contact with a surface of the image bearing member during removal of toner particles. Therefore, a deformation is caused in the edge of the cleaning member due to the friction with the image bearing member, causing stick-slip phenomenon. Thus, a micro gap is formed between the image bearing member and the cleaning blade. The smaller toner particles are more likely to get in the micro gap. The more spherical toner particles are more likely to generate rotational moment to roll within the micro gap. Toner particles rolling within the micro gap push up the cleaning blade so that toner particles can undesirably get into between the cleaning blade and the image bearing member. As a result, the micro gap is so expanded that toner particles are permitted to slip through the micro gap. Small-sized and spherical toner particles are prevented from slipping through the micro gap only when the cleaning blade is pressed against the image bearing member with a high linear pressure. However, in this case, both the image bearing member and the cleaning blade may rapidly wear due to the high pressure, shorting their lifespan against the recent demand for high durability.
On the other hand, it is known that small-sized and spherical toner particles can be removed by what is called an electrostatic cleaning system. In the electrostatic cleaning system, a conductive cleaning member (i.e., a voltage applying member) electrostatically removes toner particles from an image bearing member upon application of a voltage having the opposite polarity to the toner.
Generally, residual toner particles to be removed with the cleaning member have various charge amount. Although most toner particles on an image bearing member have a normal polarity (e.g., negative polarity) before being transferred therefrom, a part of them shifts to the opposite polarity (e.g., positive polarity) upon application of a transfer electric field having the opposite polarity to the toner particles, due to the occurrence of opposite charge injection. As a result, residual toner particles remaining on the image bearing member have an undesirable broad charge distribution in which positive and negative toner particles are coexisting.
In attempting to solve the above problem, Japanese Patent Application Publication No. 2007-272091 proposes a cleaning device having two cleaning brushes. One of the cleaning brushes is applied with a voltage having the same polarity as the toner, and the other is applied with a voltage having the opposite polarity to the toner. It is disclosed therein that the two cleaning brushes can reliably remove both positive and negative toner particles from the image bearing member. Toner particles collected by the two cleaning brushes are further electrostatically collected by metallic rollers applied with a higher absolute voltage due to the potential difference therebetween.
The predetermined resistance values of the image bearing member and the cleaning brush are generally varied with time while a cleaning voltage, having been optimized according to the predetermined resistance values, is unchangeable. Thus, the gap between the predetermined and temporal resistance values causes defective cleaning of the image bearing member. In attempting to solve this problem, Japanese Patent Application Publication No. 2009-258541 proposes a method of controlling voltage to be applied to a cleaning brush so that an optimum amount of current flows. It is disclosed therein that electrostatic cleaning ability of the cleaning brush has a high correlation with the amount of current flowing in the contact portion of the cleaning brush with the image bearing member. The proposed method keeps flowing the optimum amount of current even when the resistance values of the image bearing member and the cleaning brush are varied so that the cleaning brush can always express high cleaning ability.
In the proposed method, voltages V1 and V2 are applied to respective cleaning brushes from direct current power sources, and currents I1 and I2 flowing in the respective cleaning brushes are detected. A current-voltage characteristic graph having a vertical axis indicating voltage and a lateral axis indicating current is linearly compiled from the above values V1, V2, I1, and I2 to determine the optimum voltage to obtain the optimum current to produce the best performance of the cleaning brushes.
To meet a recent demand for more compact apparatus, two cleaning members are required to be positioned much closer. This requirement has been found to raise a new problem.
In the conventional method described above, a voltage to be applied to each of the two cleaning members is independently controlled. Specifically, a voltage to be applied to one of the cleaning members is adjusted first, and subsequently a voltage to be applied to the other is adjusted. When the voltage thus previously adjusted is applied to the cleaning members during an actual cleaning procedure, an optimum amount of current flows when the two cleaning members are positioned far away from each other. However, when the two cleaning members are positioned close to each other, an optimum amount of current does not flow during an actual cleaning procedure even when the previously adjusted voltage is applied to the cleaning members. As a result, defective cleaning occurs.
The inventors of the present invention consider the reason for such defective cleaning as follows. The image bearing member has a relatively high resistance because it is required to bear charged toner particles without degrading their charge level. When two cleaning members are separately positioned while contacting the image bearing member, the resistive component connecting the two cleaning members is very high. Therefore, the two cleaning members can be regarded as being electrically independent. When two cleaning members are closely positioned while contacting the image bearing member, the resistive component connecting the two cleaning members is very low. Therefore, the two cleaning members cannot be regarded as being electrically independent. Upon application of a voltage to one of the cleaning members, the surface potential of the image bearing member at the contact portion with the other cleaning member is increased or decreased. Since the voltage to be applied to one of the cleaning members is previously adjusted under the condition that no voltage is applied to the other cleaning member, which is different from the actual cleaning conditions, the optimum amount of current cannot flow during the actual cleaning procedure. Because voltages applied to the two cleaning members are large and opposite in polarity, a voltage applied to the one of the cleaning members has no small effect on the amount of current flows in the other.
The above-described problem may arise not only in the case in which two cleaning members are closely positioned but also in a case in which two or more voltage applying members (e.g., transfer rollers installed in a tandem image forming apparatus) are closely positioned while contacting a voltage applied member (e.g., image bearing member, recording medium conveying member).